In order to view a video signal in a state with little flicker, recently the practice of converting a video signal of the interlace method to a video signal of the progressive method has become prevalent.
A video signal of the NTSC (National Television System Committee) method is comprised of 60 fields (30 frames) per second. As opposed to this, in a signal obtained from a movie film, the number of frames per second is made 24 frames. Accordingly, when converting an image of a movie to a video signal of the NTSC method, the conversion processing is carried out by the 3-2 pull-down method. When this conversion processing is carried out, images of the same frame are arranged in two continuous fields, and the same images of the next frame are arranged in the following three continuous fields. As a result, images of 24 frames can be assigned to 60 fields.
In this way, in a 3-2 pull-down converted film video signal, the video signals of the first field and the third field among three continuous fields become exactly the same video signals. When converting a video signal of the interlace method to a video signal of the progressive method, if the same video signal has been already encoded, the processing of the video signal appearing second can be omitted. Accordingly, when it is known in advance whether or not the video signal is a film video signal, more efficient encoding becomes possible.
Therefore, a method of judging whether or not a video signal is a 3-2 pull-down converted film video signal is disclosed in for example U.S. Pat. No. 4,982,280.
FIG. 11 shows the principle of the method proposed in this U.S. Patent. As shown in the figure, a video signal consists of a signal in which odd (O) and even (E) fields alternately appear. In the case of a 3-2 pull-down converted video signal, the image (luminance signal) of the first frame A is defined as the video signal of two fields of an odd field Ao and an even field Ae.
The image of the next frame B is arranged in three fields of an odd field, an even field, and a next odd field. Namely, the first field is defined as the odd field Bo, the next field is defined as the even field Be, and the third field is defined as the odd field Bo. Accordingly, among three fields, the first odd field Bo and the third odd field Bo become exactly the same signal.
Below, video signals of frames of a film such as a frame C, a frame D, a frame E, and a frame F are assigned to fields of the video signals in the same way as above.
When now assuming that an original signal which is not delayed among the luminance signals is F0, this original signal F0 is delayed by the amount of one field to obtain a signal F1. This signal F1 is further delayed by the amount of one field to obtain a signal F2.
Looking at the value of the frame reference obtained by subtracting the signal F2 from the signal F0, as shown in FIG. 11, the values of the signals F0 and F2 become the same at Bo, De, Fo, He, . . . at least one time in each cycle consisting of five fields. As a result, the value of F0-F2 becomes 1101111011110 . . . . Namely, defining five fields as the period, the value becomes 0 at least one time in each cycle.
Contrary to this, in the case of a video signal of the ordinary NTSC method not a 3-2 pull-down converted video signal, the frame reference becomes 11111111 . . . .
Accordingly, it can be judged whether or not a video signal is a film video signal from the difference of patterns of the frame differences.
When judging if a signal is a film video signal, however, it is necessary to provide two one-field delay circuits of the one-field delay circuit for generating the signal F1 and the one-field delay circuit for generating the signal F2, so the apparatus becomes large in scale. The same problem occurs also in a case of detecting a detected signal comprised of a plurality of module signals where there is a pattern of two coinciding module signals located on two sides sandwiching a predetermined module signal at a predetermined position in a predetermined number of continuous module signals.